1. Field of the Invention
This invention relates to an impedance conversion transformer suitably employed in mixers, branching filters and matching devices associated with television antennas.
2. Discussion of the Background
In mixers, branching filters and matching devices associated with television antennas and having input and output terminals of 300 ohms, an impedance conversion transformer is employed for realizing matching between a television receiver having a coaxial cable of 75 ohms or input and output terminals of 75 ohms, a video tape recorder and parallel line feeders of 300 ohms. FIG. 1 shows one type of mixer in a block diagram. In the mixer, VHF and UHF terminals to which parallel line feeders of 300 ohms are connected are electrically connected to the tuner of an unbalanced television circuit or video tape recorder of 75 ohms through impedance conversion transformers 1 and 2, respectively.
The transformers 1 and 2, as shown in FIG. 2, are called "four-terminal balun" in the art and are used as impedance conversion transformers. The four-terminal balun comprises a pair of coils 4 wound about the walls which define two spaced holes in a spectacle-shaped core 3 and the coils are arranged in the circuit as shown in FIG. 3. The prior art such as shown in FIGS. 2 and 3, has been detailed in Unexamined Japanese Model Appln. Gazette No. 175588/1979. Although the four-terminal balun exhibits excellent characteristics even in UHF band, since the core has the spectacle shape, the size of the balun can not be reduced substantially. Furthermore the winding of the coils is also troublesome in the four-terminal balun.
In addition to the four-terminal balun type impedance conversion transformer, there is the transformer as shown in FIG. 4, which is called in the art "three-terminal balun". The three-terminal balun comprises a coil 6 wound about a toroidal core 5 with an intermediate tap b extending from an intermediate position between the opposite ends of the coil (A, a and B) and the coil is arranged in the electrical circuit as shown in FIG. 5. The prior art such as shown in FIGS. 4 and 5, is detailed in Unexamined Japanese Utility Model Appln. Gazette No. 2537/1973. The three-terminal balun can be produced having a substantially small size. Although the three-terminal balun exhibits excellent characteristics in VHF band, the balun can not exhibit practical characteristics in UHF band.